How to make PPTP VPN connect not only through a router, but also directly?

Question

VPN

There is a cable from the provider (TTK). The provider distributes the Internet simply via ethernet, without any authorization and additional protocols such as PPPoE or VPN. The cable is connected to the Zyxel Keenetic Lite router, and there are different devices connected to the router (laptop with Ubuntu 14.04, different Android devices). Has its own VPN server (PPTP, MS-CHAP v2, MPPE-128). When devices are connected to a router, it is perfectly possible to connect to a VPN server (as well as over a mobile network, for example).
But if the laptop is connected directly, not to the router, errors occur (below). Exactly the same errors occur when setting up a connection in the router itself.
So, when connected directly from a router or from a laptop, the server log looks like this:

pptpd[6304]: CTRL: Client x.x.x.x control connection started
pptpd[6304]: CTRL: Starting call (launching pppd, opening GRE)
pppd[6305]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
pppd[6305]: pptpd-logwtmp: $Version$
pppd[6305]: pppd 2.4.5 started by root, uid 0
pppd[6305]: using channel 152
pppd[6305]: Using interface ppp0
pppd[6305]: Connect: ppp0 <--> /dev/pts/1
pppd[6305]: sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0x0> <auth chap MS-v2> <magic 0x36879b0e> <pcomp> <accomp>]
pptpd[6304]: GRE: Bad checksum from pppd.
pppd[6305]: sent [LCP ConfReq id=0x1 <mru 1492> <asyncmap 0x0> <auth chap MS-v2> <magic 0x36879b0e> <pcomp> <accomp>]
pppd[6305]: last message repeated 8 times
pppd[6305]: LCP: timeout sending Config-Requests
pppd[6305]: Connection terminated.
pppd[6305]: Modem hangup
pppd[6305]: Exit.
pptpd[6304]: GRE: read(fd=6,buffer=80504c0,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
pptpd[6304]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
pptpd[6304]: CTRL: Reaping child PPP[6305]
pptpd[6304]: CTRL: Client x.x.x.x control connection finished

On the side of the router / laptop, the log contains the following:

pptp[552] control connection
pptp[552] unix_sock
pptp[553] enable echo requests (30:3)
pptp[553] Sent control packet type is 1 'Start-Control-Connection-Request'
pptp[553] Received Start Control Connection Reply
pptp[553] Client connection established.
pptp[553] Sent control packet type is 7 'Outgoing-Call-Request'
pptp[553] Received Outgoing Call Reply.
pptp[553] PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 45162 call ID (them) 3968.
pptp[553] read returned zero, peer has closed
pptp[553] Closing connection (shutdown)

Wireshark just shows that the client receives an LCP Configuration Request, but the client does not respond (judging by the log, for some reason it thinks that the request is not intended for him - the line about PPTP_OUT_CALL_RPLY). When connecting through a router, the client responds to the Configuration Request and the connection is then established.
Actually, the question is how to cure it? I really want to set up a VPN on the router.
UPD found a strange pattern:

Sep 28 19:58:47pptp[376]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54815 call ID (them) 0.
Sep 28 19:59:08pptp[380]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54817 call ID (them) 128.
Sep 28 19:59:40pptp[382]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54818 call ID (them) 256.
Sep 28 20:00:11pptp[384]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54819 call ID (them) 384.
Sep 28 20:00:42pptp[386]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54820 call ID (them) 512.
Sep 28 20:01:14pptp[389]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54822 call ID (them) 640.
Sep 28 20:01:45pptp[391]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54823 call ID (them) 768.
Sep 28 20:02:16pptp[393]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54824 call ID (them) 896.
Sep 28 20:02:48pptp[395]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54748 call ID (them) 1024.
Sep 28 20:03:19pptp[397]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54749 call ID (them) 1152.
Sep 28 20:03:50pptp[399]PPTP_OUT_CALL_RPLY received for non-existant call: peer call ID (us) 54750 call ID (them) 1280.

It's interesting how the IDs change with each request.

Answer 1

Maybe your ISP is cutting your VPN traffic?
specifically GRE protocol